Question 201089
Let {{{t}}} = time in minutes to empty tank with 
both pipes open
Given:
drain pipe rate:
(1 tankful)/(30 min)
inlet pipe rate:
(1 tankful)/(45 min)
{{{1/30 - 1/45 = 1/t}}}
Multiply both sides by {{{90t}}}
{{{3t - 2t = 90}}}
{{{t = 90}}} 
It will take 90 min to empty tank
with both pipes open.